Rat
hepatoma McA-RH7777 cells transfected with a human hepatic
lipase (HL)
cDNA synthesized and secreted 50-80 ng of human HL/mg of cell
protein at 4 h, approximately 50% of which was bound to cell-surface
heparan sulfate proteoglycans (
HSPG). The newly synthesized HL possessed enzymatic activity. When rabbit
beta-very low density lipoproteins (
beta-VLDL) and canine
chylomicrons or
chylomicron remnants were incubated with HL-secreting cells, remnant binding and uptake were enhanced 3-fold compared with nontransfected cells. Furthermore, fluorescence microscopy showed enhanced uptake of 1,1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine-labeled
beta-VLDL by the HL-transfected cells. When 125I-beta-VLDL were added to
conditioned medium from HL-secreting cells, the HL in the media enhanced the binding and uptake of the remnant
lipoproteins by nontransfected cells about 3-fold. Likewise, surface-bound HL (without HL in the medium) also was able to mediate the enhanced binding of the remnants. This HL-enhanced binding was shown to be mediated by an interaction with cell-surface
HSPG.
Heparinase treatment to remove cell-surface
HSPG or
chlorate treatment to prevent
HSPG sulfation of the HL-secreting cells abolished all the HL-mediated enhanced binding and uptake. Furthermore,
heparinase pretreatment of nontransfected cells prevented the enhanced binding and uptake of
beta-VLDL incubated with
conditioned medium from HL-secreting cells. As binding was not enhanced in the absence of
HSPG, an HL-
HSPG initial interaction appears essential. Addition of
apolipoprotein (
apo) E to the
beta-VLDL did not facilitate HL-mediated binding and uptake; in fact,
beta-VLDL from
apoE-null mice demonstrated a similar degree of enhanced binding as did rabbit
beta-VLDL with or without added
apoE. On the other hand,
beta-VLDL from transgenic mice overexpressing binding-defective
apoE(Arg142-->Cys) did not display any enhanced binding and uptake by the HL-secreting cells, and it appears that the
apoE(Arg142-->Cys) actually inhibited the HL-mediated interaction. This mutant form of
apoE is associated with a dominant mode of expression of
type III hyperlipoproteinemia in contrast to the more commonly occurring recessive disorder. Impaired HL interaction with the
apoE(Arg142-->Cys)
beta-VLDL may contribute to remnant
lipoprotein accumulation in the plasma of patients with this mutant form of
apoE. Thus, HL contributes to the enhanced cell association of specific types of remnant
lipoproteins by initiating their binding to cell-surface
HSPG.